Glycosylation of protein is not required for the export or secretion of fibronectin, acetylcholine receptor (AChR), and procollagen, nor for the fusion of embryonic muscle cells in culture. Similarly, glycosylation is not required for the biological activity of fibronectin or the alpha-bungarotoxin binding of AChR. Studies with all these systems indicated that the carbohydrate was required to stabilize the protein moiety against proteolytic degradation. The "complex" carbohydrate structure of fibronectin is synthesized as a "high-mannose" precursor.